1D Pd-Based Nanomaterials as Efficient Electrocatalysts for Fuel Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 321

cathodes. It was found that the Pt/PdNTs with Pt loading of 9 wt% (PtPd 9)
produced an ORR mass activity 95 % of PtNTs. Taking account of the reduction
of Pt loading in PtPd 9, the Pt mass activity of Pt/PdNTs is actually significantly
higher than PtNTs. Compared to the dollar activity target (9.7 A $ -1 ) of the
United States Department of Energy (DOE), the calculated dollar activity of PtPd
9 (10.4) exceeds that of DOE by 7 %. On the other hand, the area activity of Pt/
PdNTs outperformed the DOE target by greater than 40 %.
In a recent paper, Koenigsmann et al. [ 97 ] reported the synthesis, character-
ization, and electrochemical performance of novel 1D ultrathin Pt monolayer
shell-Pd nanowires core catalyst. They found that the UV ozone-treated nanowires
exhibited outstanding area and mass specific activities of 0.77 mA cm -2 and
1.83 A mg P -1 toward ORR, respectively, which were significantly enhanced as
compared with conventional commercial Pt nanoparticles, core-shell nanoparti-
cles, and acid-treated nanowires. This study also indicated that the methods to
remove the organic residue on the surface of nanomaterials can affect their sub-
sequent catalytic activity. In another report by Koenigsmann et al. [ 66 ] they
successfully synthesized a series of bimetallic Pd 1-x Au x and Pd 1-x Pt x nanowires
with control over composition and size through an ambient, template-based
technique. In their report, the as-synthesized 1D alloy nanowires (ANWs) maintain
significantly enhanced activity toward ORR as compared with commercial Pt
nanoparticles and other 1D nanostructures. Specifically, the Pd 9 Au and Pd 4 Pt
nanowires possess ORR activities of 0.49 and 0.79 mA cm -2 , which are larger
than the analogous value from commercial Pt nanoparticles (0.21 mA cm -2 ).
Recent studies have demonstrated that Pd catalysts can produce high ORR
activity when combined with appropriate transition metals, such as Co, Fe, Mo, etc
[ 59 , 61 , 98 ]. By an organic phase reaction of [Pd(acac) 2 ] and thermal decompo-
sition of [Fe(CO) 5 ] in a mixture of oleyamine and octadecene at 160 C, Li et al.
[ 73 ] synthesized PdFe nanorods (PdFe-NRs) with tunable length. The morphology
of the PdFe products can be tuned by altering the volumetric ratio of oleyamine
(OAm) and octadecene (ODE) surfactants during the synthesis. As shown in
Fig. 4 a, under the OAm/ODE ratio of 1/3, there is only PdFe nanoparticles for-
mation with an average particle size of 2-4 nm. With the ratio increasing to 1/1,
PdFe nanorods with a diameter of 3 nm and length of 10 nm were formed
(Fig. 4 b). If only OAm was used in the synthesis, long PdFe nanorods with a
length of 10 nm can be produced. Interestingly, around 20-40 PdFe nanorods were
self-assembled to ''flower''-like bundles (Fig. 4 c, d), and the distance between two
nanorods is around 2-4 nm. It was proposed that this bundle structure is likely to
form a thin and dense catalyst layer in a membrane-electrode assembly (MEA),
which can facilitate the mass transport of reactants. In the following electrocata-
lytic investigations, the as-synthesized PdFe-NRs demonstrated a better PEMFC
performance than commercial Pt/C in the practical working voltage region
(0.80-0.65 V), which can be attributed to their unique 1D morphology, high
intrinsic activity toward ORR, reduced cell inner resistance, and improved mass
transport.